U.S. patent application number 10/686697 was filed with the patent office on 2004-07-08 for polymers, methods of use thereof, and methods of decomposition thereof.
Invention is credited to Allen, SueAnn Bidstrup, Henderson, Clifford Lee, Kohl, Paul A., Wu, Xiaoqun.
Application Number | 20040132855 10/686697 |
Document ID | / |
Family ID | 32110198 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040132855 |
Kind Code |
A1 |
Kohl, Paul A. ; et
al. |
July 8, 2004 |
Polymers, methods of use thereof, and methods of decomposition
thereof
Abstract
Polymers, methods of use thereof, and methods of decomposition
thereof, are provided. One exemplary polymer, among others,
includes, a photodefinable polymer having a sacrificial polymer and
a photoinitiator.
Inventors: |
Kohl, Paul A.; (Atlanta,
GA) ; Allen, SueAnn Bidstrup; (Atlanta, GA) ;
Wu, Xiaoqun; (Wilmington, DE) ; Henderson, Clifford
Lee; (Douglasville, GA) |
Correspondence
Address: |
THOMAS, KAYDEN, HORSTEMEYER & RISLEY, LLP
100 GALLERIA PARKWAY, NW
STE 1750
ATLANTA
GA
30339-5948
US
|
Family ID: |
32110198 |
Appl. No.: |
10/686697 |
Filed: |
October 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60418930 |
Oct 16, 2002 |
|
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|
Current U.S.
Class: |
522/150 ;
522/162 |
Current CPC
Class: |
C08K 5/0041 20130101;
B81B 2203/0384 20130101; G03F 7/40 20130101; B81B 2203/0338
20130101; B81C 2201/0108 20130101; B81B 2201/058 20130101; Y10T
428/24479 20150115; G03F 7/004 20130101; B81C 1/00103 20130101;
C08J 3/28 20130101 |
Class at
Publication: |
522/150 ;
522/162 |
International
Class: |
C08J 003/28 |
Goverment Interests
[0002] The U.S. government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of MDA awarded by the National Science Foundation (Grant
#DMI-9980804) of the U.S. Government.
Claims
Therefore, having thus described the invention, at least the
following is claimed:
1. A polymer, comprising: a photodefinable polymer including a
sacrificial polymer and a photoinitiator.
2. The polymer of claim 1, wherein the photoinitiator is a negative
tone photoinitiator.
3. The polymer of claim 1, wherein the photoinitiator is a positive
tone photoinitiator.
4. The polymer of claim 1, wherein the sacrificial polymer is
selected from polynorbornenes, polycarbonates, polyethers,
polyesters, functionalized compounds of each, and combinations
thereof.
5. The polymer of claim 1, wherein the sacrificial polymer includes
polynorbornene.
6. The polymer of claim 3, wherein the polynorbornene includes
alkenyl-substituted norbornene.
7. The polymer of claim 1, wherein the photoinitiator is a free
radical generators.
8. The polymer of claim 1, wherein the photoinitiator is selected
from, bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide,
2-benzyl-2-dimethylamin- o-1-(4-morpholinophenyl)-butanone-1,
2,2-dimethoxy-1,2-diphenylethan-1-one- ,
2-methyl-1[4-(methylthio)-phenyl]-2-morpholinopropan-1-one,
2-methyl-4'-(methylthio)-2-morpholino-propiophenone, benzoin ethyl
ether, and 2,2'-dimethoxy-2-phenyl-acetophenone, and combinations
thereof.
9. The polymer of claim 1, wherein the photoinitiator is selected
from, bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide and
2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1.
10. The polymer of claim 1, wherein the sacrificial polymer is
about 1 to 30% by weight percent of the photodefinable polymer,
wherein the photoinitiator is from about 0.5 to 5% by weight of the
photodefinable polymer, wherein the solvent is about 65% to 99% by
weight percent of the photodefinable polymer.
11. A method for fabricating a structure, comprising: disposing a
photodefinable polymer onto a surface, wherein the photodefinable
polymer includes a sacrificial polymer and a photoinitiator
selected from a negative tone photoinitiator and a positive tone
photoinitiator; disposing a gray scale photomask onto the
photodefinable polymer, wherein the gray scale photomask encodes an
optical density profile defining a three-dimensional structure to
be formed from the photodefinable polymer; exposing the
photodefinable polymer through the gray scale photomask to optical
energy; and removing portions of the photodefinable polymer to form
the three-dimensional structure of cross-linked photodefinable
polymer.
12. The method of claim 11, wherein removing includes: removing
unexposed portions of the photodefinable polymer to form the
three-dimensional structure.
13. The method of claim 11, wherein removing includes: removing
exposed portions of the photodefinable polymer to form the
three-dimensional structure.
14. The method of claim 11, further comprising: disposing an
overcoat layer onto the three-dimensional structure; and
decomposing the photodefinable polymer, thermally, to form a
three-dimensional air-region.
15. The method of claim 14, wherein decomposing includes:
maintaining a constant rate of decomposition as a function of
time.
16. The method of claim 14, wherein decomposing includes:
maintaining a constant rate of mass loss of the photodefinable
polymer.
17. The method of claim 14, wherein decomposing includes: heating
the structure according to the thermal decomposition profile
expression 9 T = E a R [ ln A ( 1 - r t ) n r ] - 1 where R is the
universal gas constant, t is time, n is the overall order of
decomposition reaction, r the desired polymer decomposition rate, A
is the Arrhenius pre-exponential factor, and E.sub.a is the
activation energy of the decomposition reaction.
18. The method of claim 11, wherein the three-dimensional structure
has a spatially-varying height.
19. A structure, comprising the three-dimensional structure formed
using the method of claim 11.
20. A structure, comprising the three-dimensional air-region formed
using the method of claim 14.
21. A structure, comprising the three-dimensional air-region formed
using the method of claim 15.
22. A structure, comprising the three-dimensional air-region formed
using the method of claim 17.
23. A method of decomposing a polymer, comprising: providing a
structure having a substrate, an overcoat layer, and a polymer in a
defined area within the overcoat layer; maintaining a constant rate
of decomposition as a function of time; removing the polymer from
the area to form an air-region in the defined area.
24. The method of claim 23, wherein maintaining includes: heating
the structure according to the thermal decomposition profile
expression 10 T = E a R [ ln A ( 1 - r t ) n r ] - 1 where R is the
universal gas constant, t is time, n is the overall order of
decomposition reaction, r the desired polymer decomposition rate, A
is the Arrhenius pre-exponential factor, and E.sub.a is the
activation energy of the decomposition reaction.
25. A structure, comprising: a substrate; an air-region area having
a spatially-varying height; and an overcoat layer disposed onto a
portion of the substrate and engaging a substantial portion of the
air-region area.
26. The structure of claim 25, wherein the air-region area has a
non-rectangular cross-section.
27. The structure of claim 25, wherein the air-region area has an
asymmetrical cross-section.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to co-pending U.S.
provisional application entitled "Fabrication of Microchannels
using Polynorbornene Photosensitive Sacrificial Materials" having
ser. No.60/418,930, filed on Oct. 16, 2002, which is entirely
incorporated herein by reference.
TECHNICAL FIELD
[0003] The present invention is generally related polymers, and,
more particularly, is related to photodefinable polymers, methods
of use thereof, and methods of decomposition thereof.
BACKGROUND
[0004] Microfluidic devices have tremendous potential for
applications in a variety of fields including drug discovery,
biomedical testing, and chemical synthesis and analysis. In such
devices, liquids and gases are manipulated in microchannels with
cross-sectional dimensions on the order of tens to hundreds of
micrometers. Processing in such microchannel devices offers a
number of advantages including low reagent and analyte consumption,
highly compact and portable systems, fast processing times, and the
potential for disposable systems. However, in spite of all of their
promise, microfluidic devices are currently being used in a limited
number of applications and are in general still rather simple
devices in terms of their operational complexity and capabilities.
For example, in terms of making truly portable microanalytical
systems, one of the current difficulties involves the simple
integration of electronic (e.g., sensing methods) and fluidic
elements into the same device. One of the most important issues,
which controls this ability to integrate functions into the same
device, and thus controls the level of functionality of a
microfluidic device is, the method used to fabricate the structure.
In addition, fluid microdynamics through the microchannels is
important to avoid mixing in systems where mixing is not
needed.
[0005] The two most prevalent methods for fabricating microfluidic
devices to date involve either bonding together layers of ultraflat
glass or elastomeric polymers such as poly(dimethylsiloxane). Both
methods suffer from severe limitations and difficulties associated
with integrating non-fluidic elements such as detectors with the
microchannel system in the same substrate. Other methods suffer
from several limitations including the fact that they require on
the order of ten processing steps to complete the sequence for a
single level of microchannels.
SUMMARY OF THE INVENTION
[0006] Briefly described, embodiments of this disclosure, among
others, include polymers, methods of use thereof, and methods of
decomposition thereof. One exemplary polymer, among others,
includes a photodefinable polymer having a sacrificial polymer and
a photoinitiator.
[0007] Methods of for fabricating a structure are also provided.
One exemplary method includes, among others: disposing a
photodefinable polymer onto a surface, wherein the photodefinable
polymer includes a sacrificial polymer and a photoinitiator
selected from a negative tone photoinitiator and a positive tone
photoinitiator; disposing a gray scale photomask onto the
photodefinable polymer, wherein the gray scale photomask encodes an
optical density profile defining a three-dimensional structure to
be formed from the photodefinable polymer; exposing the
photodefinable polymer through the gray scale photomask to optical
energy; and removing portions of the photodefinable polymer to form
the three-dimensional structure of cross-linked photodefinable
polymer.
[0008] In addition, methods of decomposing a polymer are also
provided. One exemplary method includes, among others: providing a
structure having a substrate, an overcoat layer, and a polymer in a
defined area within the overcoat layer; maintaining a constant rate
of decomposition as a function of time; and removing the polymer
from the area to form an air-region in the defined area.
[0009] Furthermore, a structure is provided. One exemplary
structure includes a substrate; an air-region area having a
spatially-varying height; and an overcoat layer disposed onto a
portion of the substrate and engaging a substantial portion of the
air-region area.
[0010] Other systems, methods, features, and advantages will be, or
become, apparent to one with skill in the art upon examination of
the following drawings and detailed description. It is intended
that all such additional systems, methods, features, and advantages
be included within this description, be within the scope of the
present invention, and be protected by the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Many aspects of this disclosure can be better understood
with reference to the following drawings. The components in the
drawings are not necessarily to scale, emphasis instead being
placed upon clearly illustrating the principles of this disclosure.
Moreover, in the drawings, like reference numerals designate
corresponding parts throughout the several views.
[0012] FIG. 1 illustrates representative embodiments of
photoinitiators.
[0013] FIG. 2 illustrates a cross-sectional view of a
representative structure having an embodiment on an air-region.
[0014] FIGS. 3A through 3F are cross-sectional views that
illustrate a representative method of fabricating the structure
illustrated in FIG. 2.
[0015] FIGS. 4A through 4D illustrate the cross sections of the
four simulated channels. FIG. 4A illustrates the dimensions of a
uniform area channel. FIGS. 4B and 4C illustrate channels with
tapered corners.
[0016] FIGS. 5A through 5C illustrate plots of the transit times
for fluid packets as a function of radial distance along the corner
for a standard rectangular channel geometry turn, a triangular
cross section channel turn, and an improved channel turn in a
structure, respectively.
[0017] FIG. 6 illustrates curves of the decomposition rate versus
time for pure polynorbornene (PNB) samples decomposed at both a
constant temperature of 425.degree. C. (isothermal decomposition)
and various heating rates (dynamic decomposition),
respectively.
[0018] FIG. 7 illustrates the temperature versus time heating
profiles required to achieve decomposition rates of 1, 2, and 3%
per minute using equation (6) in Example 1.
[0019] FIG. 8 illustrates the temperature versus time curve
calculated using equation (6) and the corresponding simple mimic
heating profile that was tested in the Lindberg decomposition
furnaces for device fabrication.
[0020] FIG. 9 illustrates thermogravimetric analysis (TGA) results
for the simple mimic heating program that was designed to achieve a
1% per minute decomposition rate.
[0021] FIGS. 10A through 10G illustrate scanning electron
microscope (SEM) images of the channel encapsulated with polyimide
and decomposed at different rates using different heating
profiles.
[0022] FIGS. 11A through 11F illustrate SEM images of channels
encapsulated with SiO.sub.2.
[0023] FIG. 12 illustrates the contrast curves for two
photosensitive polymer formulations used in Example 1.
[0024] FIGS. 13 and 14 illustrate the real Feature I type PNB
patterns produced as measured by profilometry, and for comparison
the predicted microchannel patterns (using equations 7 through 10
in Example 1), for the systems with 2 wt % and 4 wt % initiator
loadings.
[0025] FIGS. 15A through 15D illustrates SEM images of the tapered
microchannels.
[0026] FIG. 16 illustrates the predicted transit times for flow
around a microchannel corner using the boundary conditions and
velocities used in the earlier idealized channel simulations.
DETAILED DESCRIPTION
[0027] In general, polymer, methods of use thereof, structures
formed therefrom, and methods of decomposition thereof, are
disclosed. Embodiments of the polymer can be used to form
photodefinable three-dimensional structures having unique spatial
dimensions (e.g., spatially-varying height) using photolithographic
techniques. In addition, methods of decomposition can be used to
decompose the polymer three-dimensional structure located within a
material (e.g., an overcoat layer) without altering (e.g.,
deforming) the spatial boundaries defined by the photodefinable
polymer three-dimensional structure.
[0028] Embodiments of the polymer include a photodefinable polymer.
The photodefineable polymer includes, but is not limited to, one or
more sacrificial polymers and one or more photoinitiators. The
photoinitiator can include a negative tone photoinitiator and/or a
positive tone photoinitiator.
[0029] In general, negative tone photoinitiators can be used making
the sacrificial polymer more difficult to remove (e.g., more stable
towards a solvent that normally would dissolve the sacrificial
polymer). For example, half of a layer of a photodefinable polymer
(including a sacrificial polymer and a negative tone
photoinitiator) is exposed to optical energy (e.g., ultraviolet
(UV) light, near-ultraviolet light, and/or visible light), while
the other half is not exposed. Subsequently, the entire layer is
exposed to a solvent and the solvent dissolves the layer not
exposed to the UV light.
[0030] More specifically, the area exposed includes a cross-linked
photodefinable polymer, while portions not exposed include an
uncross-linked photodefinable polymer. The uncross-linked
photodefinable polymer can be removed with the solvent leaving the
cross-linked photodefinable polymer (e.g., the photodefinable
three-dimensional structure).
[0031] Although not intending to be bound by theory, upon exposure
to optical energy, one type, among others, of the negative tone
photoinitiator can generate free radicals that initiate
cross-linking reactions between the sacrificial polymers to form a
cross-linked photodefinable polymer. As a result, gray scale
lithography can be used to fabricate photodefinable
three-dimensional structures from the photodefinable polymer by
removing the uncross-linked photodefinable polymer.
[0032] In general, positive tone photoinitiators can be used making
the sacrificial polymer easier to remove (e.g., less stable towards
a solvent). For example, half of a layer of a photodefinable
polymer (including a sacrificial polymer and a positive tone
photoinitiator) is exposed to UV light, while the other half is not
exposed. Subsequently, the entire layer is exposed to a solvent and
the solvent dissolves the layer exposed to the UV light.
[0033] Although not intending to be bound by theory, upon exposure
to optical energy, the positive tone photoinitiator generates an
acid. Then, upon exposure to a base, the dissolution of the
sacrificial polymer is increased relative to sacrificial polymer
not exposed to optical energy. As a result, gray scale lithography
can be used to fabricate photodefinable three-dimensional
structures from the photodefinable polymer by removing the exposed
photodefinable polymer.
[0034] In general, the photodefinable polymer can be used in areas
such as, but not limited to, microelectronics (e.g., microprocessor
chips, communication chips, and optoeletronic chips),
microfluidics, sensors, analytical devices (e.g.,
microchromatography), as a sacrificial material to create
photodefinable three-dimensional structures that can be
subsequently formed into photodefinable air-regions by thermally
decomposing the photodefinable polymer. In addition, the
photodefinable polymer can be used as an insulator, for
example.
[0035] For embodiments using the photodefinable polymer as a
sacrificial material to create photodefinable air-regions having
photodefinable three-dimensional structures, the decomposition of
the photodefinable polymer should produce gas molecules small
enough to permeate one or more of the materials surrounding the
photodefinable polymer (e.g., an overcoat polymer layer). In
addition, the photodefinable polymer should slowly decompose so as
to not create undue pressure build-up while forming the air-region
within the surrounding materials. Furthermore, the photodefinable
polymer should have a decomposition temperature less than the
decomposition or degradation temperature of the surrounding
material. Still further, the photodefinable polymer should have a
decomposition temperature above the deposition or curing
temperature of an overcoat material but less than the degradation
temperature of the components in the structure in which the
photodefinable polymer is being used.
[0036] The sacrificial polymer can include compounds such as, but
not limited to, polynorbornenes, polycarbonates, polyethers,
polyesters, functionalized compounds of each, and combinations
thereof. The polynorbornene can include, but is not limited to,
alkenyl-substituted norbornene (e.g., cyclo-acrylate norbornene).
The polycarbonate can include, but is not limited to, norbornene
carbonate, polypropylene carbonate, polyethylene carbonate,
polycyclohexene carbonate, and combinations thereof. In addition,
the molecular weight of the sacrificial polymer should be between
10,000 and 200,000.
[0037] The sacrificial polymer can be from about 1% to 30% by
weight of the photodefinable polymer. In particular, the
sacrificial polymer can be from about 5% to 15% by weight of the
photodefinable polymer.
[0038] As mentioned above, the photoinitiator can include negative
tone photoinitiators and positive tone photoinitiators. The
negative tone photoinitiator can include compounds that generate a
reactant that would cause the crosslinking of the sacrificial
polymer. The negative tone photoinitiators can include compounds,
such as, but not limited to, a photosensitive free radical
generator. Alternative negative tone photoinitiators can be used
such as photoacid generators (e.g., in an epoxide functionalized
systems).
[0039] A negative tone photosensitive free radical generator is a
compound which, when exposed to light breaks into two or more
compounds, at least one of which is a free radical. In particular,
the negative tone photoinitiator can include, but is not limited
to, bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Structure 1
in FIG. 1) (Irgacure 819, Ciba Specialty Chemicals Inc.),
2-benzyl-2-dimethylamino-1- -(4-morpholinophenyl)-butanone-1
(Structure 2 in FIG. 1) (Irgacure 369, Ciba),
2,2-dimethoxy-1,2-diphenylethan-1-one (Structure 3 in FIG. 1)
(Irgacure 651, Ciba),
2-methyl-1[4-(methylthio)-phenyl]-2-morpholinopropa- n-1-one
(Structure 4 in FIG. 1) (Irgacure 907, Ciba), benzoin ethyl ether
(Structure 5 in FIG. 1) (BEE, Aldrich),
2-methyl-4'-(methylthio)-2-morpho- lino-propiophenone,
2,2'-dimethoxy-2-phenyl-acetophenone (Irgacure 1300, Ciba), and
combinations thereof. In particular, the photoinitiator can include
bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide and
2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1.
[0040] The positive tone photoinitiators can include, but are not
limited to, photoacid generators. More specifically, the positive
tone photoinitiator can include, but is not limited to,
nucleophilic halogenides (e.g., diphenyliodonium) and complex metal
halide anions (e.g., triphenylsulphonium salts).
[0041] The photoinitiator can be from about 0.5% to 5% by weight of
the photodefinable polymer. In particular, the photoinitiator can
be from about 1% to 3% by weight of the photodefinable polymer.
[0042] The remaining percentage of the photodefinable polymer not
accounted for in the photoinitiator and sacrificial polymer (e.g.,
from 65% about to 99%) to can be made up with solvent, such as, but
not limited to, mesitylene MS, N-methyl-2-pyrrolidinone,
propyleneglycol monomethyl ether acetate, N-butyl acetate diglyme,
ethyl 3-ethoxypropionate, and combinations thereof.
[0043] Exemplary photodefinable polymers include those shown in
Table 1.
1TABLE 2 Conditions and results for UV exposure response, with
different photoinitiators and the loading of Irgacure 819 Recipe of
PNB/PI Photo- Solution sensitivity Experiment# Photoinitiator
PNBI/PI/MS (wt %) (mJ/cm.sup.2) Contrast 1 BEE 16/0.64/83.36 1959
0.908 2 Irgacure 907 16/0.64/83.36 3641 0.651 3 Irgacure 651
16/0.64/83.36 1054 0.907 4 Irgacure 369 16/0.64/83.36 1808 0.521 5
Irgacure 819 16/0.64/83.36 134 1.213 6 Irgacure 819 16/0.32/83.68
363 0.879 7 Irgacure 819 16/0.16/83.84 3236 0.448 Processing
conditions: Spin-coating/2400 rpm, Softbake/110.degree. C., 1 min,
PEB/120.degree. C., 30 min, Developer/xylene.
[0044] Now having described the photodefinable polymer in general,
the following describes exemplar embodiments for using the
photodefinable polymer to produce photodefinable three-dimensional
structures, where the photodefinable three-dimensional structures
can be decomposed to form photodefinable air-regions (e.g., a gas
filled region substantially excluding a solid or liquid material or
a vacuum-region).
[0045] In general, a photodefinable three-dimensional structure can
be produced by disposing a layer of the photodefinable polymer onto
a substrate and/or layer of material on the substrate. A gray scale
photomask is disposed onto the photodefinable polymer or portions
thereof that encodes the photodefinable three-dimensional
structure, as described below. After exposing the photodefinable
polymer through the gray scale photomask to optical energy and
removing the unexposed photodefinable polymer (negative tone) or
the exposed photodefinable polymer (positive tone), the
photodefinable three-dimensional structure is formed.
[0046] The gray scale photomask encodes an optical density profile
that defines the three-dimensional photodefinable structure. Upon
exposure of the gray scale photomask to optical energy, a known
amount of optical energy is allowed to pass through portions of the
gray scale photomask. The design of the gray scale photomask is
used to control the amount of optical energy allowed to pass
through the gray scale photomask. In particular, the gray scale
photomask can be designed to control the amount of optical energy
allowed to pass through the gray scale photomask as a function of
the position on the gray scale photomask. Thus, the gray scale
photomask can be designed and used to produce the three-dimensional
structure from the photodefinable polymer by altering the amount of
optical energy allowed to pass through the gray scale photomask as
a function of the position on the gray scale photomask. The gray
scale photomask can be formed by method known in the art (U.S. Pat.
No. 4,622,114).
[0047] The three-dimensional structures (and the corresponding
photodefinable air-regions) can have cross-sectional areas section
such as, but not limited to, non-rectangular cross-sections,
asymmetrical cross-sections, curved cross sections, arcuate cross
sections, tapered cross sections, cross sections corresponding to
an ellipse or segment thereof, cross sections corresponding to a
parabola or segment thereof, cross sections corresponding to a
hyperbola or segment thereof, and combinations thereof. For
example, the three-dimensional structures can include, but are not
limited to, non-rectangular structures, non-square structures,
curved structures, tapered structures, structures corresponding to
an ellipse or segment thereof, structures corresponding to a
parabola or segment thereof, structures corresponding to a
hyperbola or segment thereof, and combinations thereof. In
addition, the three-dimensional structures can have cross-sectional
areas having a spatially-varying height.
[0048] FIG. 2 is a cross-sectional view of a representative
non-rectangular, tapered, and asymmetrical photodefinable
air-region 12 having a photodefinable three-dimensional structure.
For example, the non-rectangular, tapered, and asymmetrical
photodefinable air-region 12 can be used as a corner section in a
microfluidic system. This use, as well as others, is described in
more detail in Example 1.
[0049] As shown in FIG. 2, the non-rectangular, tapered, and
asymmetrical photodefinable air-region 12 is positioned on a
substrate 10. An overcoat polymer layer 14 is disposed around the
non-rectangular, tapered, and asymmetrical photodefinable
air-region 12. In another embodiment, among others, the
non-rectangular, tapered, and asymmetrical photodefinable
air-region 12 can be positioned above the substrate 10 in the
overcoat layer 14. In still another embodiment, among others, the
multiple non-rectangular, tapered, and asymmetrical photodefinable
air-regions and other air-regions can be positioned at multiple
heights (e.g., stacked on top of one another or stacked in an
offset manner) in the overcoat layer 14.
[0050] Although not illustrated, the non-rectangular, tapered, and
asymmetrical photodefinable air-region 12 can be formed in
conjunction with other air-regions and/or air-channels to form
microfluidic devices, sensors, and analytical devices, for
example.
[0051] The substrate 10 can be used in systems such as, but not
limited to, microprocessor chips, microfluidic devices, sensors,
analytical devices, and combinations thereof. Thus, the substrate
10 can be made of materials appropriate for the system. However,
exemplar materials include, but are not limited to, glasses,
silicon, silicon compounds, germanium, germanium compounds,
gallium, gallium compounds, indium, indium compounds, or other
semiconductor materials and/or compounds. In addition, the
substrate 10 can include non-semiconductor substrate materials,
including any dielectric material, metals (e.g., copper and
aluminum), or ceramics or organic materials found in printed wiring
boards, for example.
[0052] The overcoat polymer layer 14 can be a modular polymer that
includes the characteristic of being permeable or semi-permeable to
the decomposition gases produced by the decomposition of a
sacrificial polymer while forming the non-rectangular, tapered, and
asymmetrical photodefinable air-region 12. In addition, the
overcoat polymer layer 14 has elastic properties so as to not
rupture or collapse under fabrication and use conditions. Further,
the overcoat polymer layer 14 is stable in the temperature range in
which the photodefinable polymer decomposes.
[0053] Examples of the overcoat polymer layer 14 include compounds
such as, for example, polyimides, polynorbornenes, epoxides,
polyarylenes ethers, parylenes, inorganic glasses, and combinations
thereof. More specifically the overcoat polymer layer 14 includes
compounds such as Amoco Ultradel.TM. 7501, BF Goodrich Avatrel.TM.
Dielectric Polymer, DuPont 2611, DuPont 2734, DuPont 2771, DuPont
2555, silicon dioxide, silicon nitride, and aluminum oxide. The
overcoat polymer layer 14 can be deposited onto the substrate 10
using techniques such as, for example, spin coating,
doctor-blading, sputtering, lamination, screen or stencil-printing,
chemical vapor deposition (CVD), and plasma-based deposition
systems.
[0054] The non-rectangular, tapered, and asymmetrical
photodefinable air-region 12 is formed by the removal (e.g.,
decomposition) of a crosslinked photodefinable polymer (a negative
tone photoinitiator) from a defined non-rectangular, tapered, and
asymmetrical area as illustrated in FIG. 2.
[0055] It should be noted that additional components could be
disposed on and/or within the substrate, the overcoat layer, and/or
the non-rectangular, tapered, and asymmetrical photodefinable
air-region 12. In addition, the additional components can be
included in any structure having air-regions as described herein.
The additional components can include, but are not limited to,
electronic elements (e.g., switches and sensors), mechanical
elements (e.g., gears and motors), electromechanical elements
(e.g., movable beams and mirrors), optical elements (e.g., lens,
gratings, and mirror), opto-electronic elements, fluidic elements
(e.g., chromatograph and channels that can supply a coolant), and
combinations thereof.
[0056] Although the spatial boundaries of the non-rectangular,
tapered, and asymmetrical photodefinable air-region 12 are not
easily defined because of the varying lengths, heights, and widths
of the air-region, the following spatial boundaries are provided as
exemplary lengths, heights, and widths. The non-rectangular,
tapered, and asymmetrical photodefinable air-region 12 height can
range from about 0.01 to about 100 micrometers. The
non-rectangular, tapered, and asymmetrical photodefinable
air-region 12 width can be from about 0.01 to about 10,000
micrometers. The non-rectangular, tapered, and asymmetrical
photodefinable air-region 12 length can range from 0.01 micrometers
about 100 meters. It should be noted that a plurality of
air-regions can be formed such that larger and/or more intricate
(e.g., multiple curves in the x-, y-, and z-planes) air-regions can
be formed.
[0057] FIGS. 3A through 3F are cross-sectional views that
illustrate a representative process for fabricating the
non-rectangular, tapered, and asymmetrical photodefinable
air-region 12 illustrated in FIG. 2. It should be noted that for
clarity, some portions of the fabrication process are not included
in FIGS. 3A through 3F. As such, the following fabrication process
is not intended to be an exhaustive list that includes all steps
required for fabricating the non-rectangular, tapered, and
asymmetrical photodefinable air-region 12. In addition, the
fabrication process is flexible because the process steps may be
performed in a different order than the order illustrated in FIGS.
3A through 3F or some steps may be performed simultaneously.
[0058] FIG. 3A illustrates the substrate 10 having the
photodefinable polymer 16 (negative tone) disposed thereon. The
photodefinable polymer 16 can be deposited onto the substrate 10
using techniques such as, for example, spin coating,
doctor-blading, sputtering, lamination, screen or stencil-printing,
melt dispensing, chemical vapor deposition (CVD), and plasma-based
deposition systems.
[0059] FIG. 3B illustrates a gray scale photomask 18 disposed on
the photodefinable polymer 16. The gray scale photomask 18 encodes
an optical density profile that defines to the cross-section of the
non-rectangular, tapered, and asymmetrical photodefinable
air-region 12.
[0060] FIG. 3C illustrates the uncross-linked photodefinable
polymer region 16A and the cross-linked photodefinable polymer
region 16B after exposure of the gray scale photomask 18 to optical
energy, while FIG. 3D illustrates the removal of the uncross-linked
photodefinable polymer region 16A. The uncross-linked
photodefinable polymer region 16A can be removed by dissolution in
a liquid, such as a solvent, for example, or by another method that
can remove or dissolve the polymer.
[0061] FIG. 3E illustrates the formation of the overcoat layer 14
onto the cross-linked photodefinable polymer region 16B. The
overcoat layer 14 can be deposited onto the substrate using
techniques such as, for example, spin coating, doctor-blading,
sputtering, lamination, screen or stencil-printing, melt
dispensing, chemical vapor deposition (CVD), and plasma-based
deposition systems.
[0062] FIG. 3F illustrates the decomposition of the cross-linked
photodefinable polymer region 16B to form the non-rectangular,
tapered, and asymmetrical photodefinable air-region 12. The
cross-linked photodefinable polymer region 16B can be decomposed by
heating the cross-linked photodefinable polymer 16B to a
temperature sufficient to decompose the polymer (e.g., about
425.degree. C.).
[0063] The thermal decomposition the photodefinable polymer
(cross-linked photodefinable polymer in FIGS. 3A through 3F) can
alter the spatial boundaries or dimensions of the resultant
air-region (non-rectangular, tapered, and asymmetrical
photodefinable air-region 12 shown in FIG. 2) if the photodefinable
polymer decomposes too fast. As discussed in greater detail in
Example 1, the thermal decomposition of the photodefinable polymer
can cause the air-region to bubble and/or collapse (e.g., sag) in
one or more areas of the air-region. Alteration of the spatial
boundaries of the cross-section can cause problems for systems
where known and designed cross-sections are necessary for the
system to function properly.
[0064] For example, fluidic systems often need to have a known flow
profile to ensure mixing is or is not occurring. If the channels in
the fluidic system have regions with unknown cross-sections and/or
cross-sections not conforming to the design, the fluid flowing
through the channel may have an unknown and an unpredictable flow
profile.
[0065] Embodiments of this disclosure provide thermal decomposition
profiles that substantially eliminate alterations to the spatial
boundaries of the air-region caused by the decomposition of the
polymer (e.g., sacrifcial polymers and photodefinable polymers).
Prior solutions included using a constant temperature to decompose
the polymer, while others used linear temperature profiles to
decompose the polymer. Problems associated with both of these are
described in more detail in Example 1.
[0066] Embodiments of this disclosure describe decomposing the
polymer at a constant rate of decomposition versus time. Thermal
decomposition profiles based on maintaining a constant
decomposition rate as a function of time can substantially
eliminate alterations of the spatial boundaries of the air-region.
In other words, the decomposition is performed at a constant rate
of mass loss (grams per minute) of the photodefinable polymer.
[0067] Thermal decomposition profiles can be expressed by the
thermal decomposition profile expression (equation 6 in the
following Example). 1 T = E a R [ ln A ( 1 - r t ) n r ] - 1
[0068] where R is the universal gas constant, t is time, n is the
overall order of decomposition reaction, r the desired polymer
decomposition rate, A is the Arrhenius pre-exponential factor, and
E.sub.a is the activation energy of the decomposition reaction.
Thus, in order to design a thermal decomposition profile it is
helpful to specify four parameters: the three kinetic parameters
(A, E.sub.a and n) that describe the polymer decomposition for each
polymer, and r the desired polymer decomposition rate. Example 1
describes the thermal decomposition profile expression in greater
detail.
[0069] It should be noted that not all thermal decomposition
profiles produce decomposition of the polymer that do not alter the
spatial boundaries of the air-region. Example 1 includes an
illustrative polymer where thermal decomposition profiles greater
than about 2% decomposition/minute alter the spatial boundaries of
the air-region, while thermal decomposition profiles below about 2%
decomposition/minute do not alter the spatial boundaries of the
air-region. Therefore, one skilled in the art could easily
experimentally determine the appropriate thermal decomposition
profile through a sequence of experiments without undue
experimentation.
[0070] One example, among others, for experimentally determining
the appropriate thermal decomposition profile includes starting
with a 5% decomposition/minute profile. If the spatial boundaries
of the air-region are altered, then the thermal decomposition
profile can be reduced to a 4% decomposition/minute profile or 2.5%
decomposition/minute profile, for example. Alternatively, if the
spatial boundaries of the air-region are not altered, then the
thermal decomposition profile can be increase by a 1%
decomposition/minute profile or more, for example (i.e., form 5%
decomposition/minute profile to 6% decomposition/minute profile).
In any event, one skilled in the art can use the teachings of this
disclosure to obtain an appropriate thermal decomposition profile
for numerous desired configurations.
[0071] It should also be noted that the thermal decomposition
profile could depend upon a variety of factors such as, for
example, the materials surrounding the photodefinable polymer, the
hardness of the overcoat, and/or the glass transition temperature
of the overcoat. Thus, these variables can be considered in the
selection of the thermal decomposition profile.
EXAMPLE 1
[0072] The following is a non-limiting illustrative example of an
embodiment of this disclosure that is described in more detail in
Wu, et al., Journal of the Electrochemical Society, 150, 9,
H205-H213 (2003), which is incorporated herein by reference. This
example is not intended to limit the scope of any embodiment of
this disclosure, but rather is intended to provide specific
exemplary conditions and results. Therefore, one skilled in the art
would understand that many conditions can be modified to produce a
desired result, and it is intended that these modifications be
within the scope of the embodiments of this disclosure. In
addition, additional details related to this example can be found
in Wu, et al., Journal of the Electrochemical Society, 149, 10,
G555-G561 (2002) and Wu, et al., J. Appl. Polym. Sci, 88, 5,
1186-1195 (2003), both of which are incorporated herein by
reference.
[0073] Example 1 describes the development of and demonstrates the
use of photodefinable sacrificial polymer fabrication methods to
produce channel geometries with non-rectangular, tapered, and
asymmetrical shaped cross-sectional profiles. The ability to
control the shape of the channel cross-section is expected to be
particularly useful in precisely controlling the flow of fluids in
microchannel systems, for example. The ability to control fluid
flow patterns and dispersion by controlling the channel cross
section is investigated herein through computational fluid dynamics
simulations. It was found that non-rectangular, tapered, and
asymmetrical shaped cross-sectional channel profiles are useful in
preserving "plug flow" conditions in curved microchannels, for
example, and thus reducing dispersion of components in the flow.
Therefore, the thermal decomposition of the photodefinable
sacrificial polymers was studied in detail and novel heating
protocols were developed that maintain the channel shape during
decomposition. The use of these methods demonstrated using gray
scale lithography to produce microchannels with tapered cross
sections.
[0074] Simulation of Flow in Curved Channels
[0075] When designing and fabricating microfluidic devices, it is
almost inevitable that channels with curved shapes are needed. For
example, when designing a long separation column on a chip, turning
the channel into a meandering path may be required to keep the
device within some required size limits. In such cases, it can be
extremely important to precisely control the fluid flow pattern in
the channel so as to minimize differences in the residence time
distribution of fluid traveling through the channel. In other
words, one generally would like to maintain near "plug flow"
conditions in devices used for separations, analysis, and other
fluidic operations to prevent mixing and loss of spatial
confinement of fluid samples after injection or separation. One
particular problem is minimizing residence time variations for
fluids traveling through corners and curved sections of
microfluidic channels. In order to illustrate this point and
investigate the improvements that could be realized by using
channels with tapered cross sections, a series of computational
fluid dynamics simulations were performed.
[0076] FLUENT.TM., a computational fluid dynamics (CFD) simulation
package produced by Fluent Inc., was used to simulate the flow in a
series of different corner designs for microchannels. GAMBIT.TM., a
preprocessor accessory for FLUENT made by Fluent Inc., was used to
construct the desired model geometry, apply the meshing points to
the model, and define the required boundary zones. Once defined,
FLUENT was used to simulate the flow pattern in each microchannel
and to produce numerical and graphical results for each case.
[0077] A series of 90 degree turns in microchannels were simulated
with varying cross sectional geometries. FIGS. 4A through 4D
illustrates the cross sections of the four simulated channels. FIG.
4A illustrates the dimensions of a uniform area channel. FIGS. 4B
and 4C illustrate channels with tapered corners. The taper improved
the flow around corners with FIG. 4D representing a near optimized
design. The inside radius of the turns was held constant at 60
.mu.m and the outside radius of the turns was held constant at 120
.mu.m. The same boundary conditions were applied in simulating the
flow through these channels and a constant pressure outlet
condition, which was assumed to be atmospheric pressure. In this
case, water was used as the flow media, but the results should be
general to any Newtonian fluid under laminar flow conditions. Under
these conditions, the flow rates and Reynolds numbers are quite low
which indicates laminar flow conditions, and thus a laminar flow
model was used in FLUENT for solution of these problems.
[0078] In order to look at the dispersion, which would occur in
fluid flowing around each of these microchannel corners, fluid
packet trajectories and transit times around each turn were
calculated. FIGS. 5A through 5C illustrate plots of the transit
times for fluid packets as a function of radial distance along the
corner for a standard rectangular channel geometry turn, a
triangular cross section channel turn, and an improved channel turn
in a structure, respectively. The appearance of different lines of
packet transit times on these plots are due to the fact that fluid
packets at different vertical positions within the channels also
experience slight dispersion due to low velocities near the top and
bottom surfaces of the channel.
[0079] FIG. 5A illustrates that the standard rectangular channel
geometry would result in severe dispersion of an initially flat
concentration profile after traveling around the 90 degree turn.
Under laminar flow conditions, the relatively uniform velocity
profile across the channel cross section coupled with the longer
path length for fluid at the outside of the turn result in transit
times, which are a factor of 3 to 5 larger for fluid at the outside
radius as compared to fluid at the inside radius. This dispersion
would be even more greatly exaggerated for the case of a 180 degree
bend.
[0080] One natural solution to this problem is to decrease the
velocity of the fluid near the inside radius of the turn in order
to achieve equal transit times for the fluid irrespective of radial
position. One way to achieve this velocity modification is to alter
the cross sectional area of different regions of the channel.
[0081] FIG. 5B illustrates the transit time profile for fluid
flowing around a turn in a triangular cross section channel. The
reduced channel height at the inside radius of the turn would be
expected to slow the velocity of fluid along the inside of the
turn. Indeed, FIG. 5B shows that the triangular cross section
overcompensates and results in longer transit times for fluid near
the inside radius as compared to fluid near the outside radius.
From close inspection of the FLUENT results, it is also apparent
that channel sections that have two walls intersecting at acute
angles leads to significant dispersion in these regions, and thus
acute angles in the channel cross section geometry should be
avoided if possible. Based on these facts, an optimization was
performed to design an improved channel cross section profile that
would result in minimal dispersion around the simulated turn.
[0082] FIG. 5C illustrates the fluid transit time results for such
an improved channel structure. The transit time profile is
essentially uniform for fluid flowing around a 90 degree corner
using this improved shape. Thus, it is clear that by designing the
cross-sectional shape of microchannels in turns it should be
possible to minimize dispersion in the flow profiles.
[0083] Experimental
[0084] The sacrificial polymer used was Unity.TM. 4481P, which
includes the copolymer of 5-butyl norbornene (BuNB) and 5-alkenyl
norbornene (ANB) in the molar ratio 73/27, (Promerus LLC,
Brecksville, Ohio). The polymer weight average molecular weight
(M.sub.W) and polydispersity index (PDI) were measured to be
425,000 and 3.74, respectively, by gel permeation chromatography
using polystyrene calibration standards.
Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819,
Ciba Specialty Chemicals Inc.) was used as a free radical
photoinitiator (PI). Solutions of polynorbornene (PNB) and PI were
prepared using mesitylene (MS, 97%, Aldrich) as the solvent. Two
different formulations, PNB/PI/MS in a mole ratio of 16/0.32/84 (2
wt % initiator relative to dry polymer) and PNB/PI/MS in a mole
ratio of 16/0.64/84 (4 wt % initiator relative to dry polymer)
(weight ratios), were used in the experiments. After exposure and
baking, polymer patterns were developed using xylene (98.5+%,
Aldrich).
[0085] Thermal decomposition characteristics of the sacrificial
polymer were investigated using a Seiko Instruments Inc. TG/DTA 320
system. Thermogravimetric analysis (TGA) measurements were
performed under N.sub.2 at a purge rate of 28 milliliters/minute
(mil/min). The encapsulated sacrificial polymer structures were
thermally decomposed in a Lindberg tube furnace purged with
N.sub.2.
[0086] For microchannel fabrication, PNB/PI films were cast onto
silicon wafers using a Brewer Science CEE 100 spinner and hotplate
system. About a 3.5 to 4.0 micrometer (.mu.m) thick PNB/PI film was
obtained at a spin speed of 2400 revolutions per minute (rpm) and a
softbake of 110.degree. C. for 60 seconds (s). Film thicknesses
were measured using a Veeco Dektak profilometer. An OAI Mask
Aligner equipped with an i-line filtered UV irradiation source (365
nanometers (nm) wavelength) was used to expose and pattern the
PNB/PI films. Before exposure, the intensity of UV light source was
measured using an OAI Model 356 Exposure Analyzer with a 365 nm
probe. After exposure, samples were post-exposure baked at
120.degree. C. for 30 minutes in an oven. Samples were developed
using a continuous spray of xylene while the wafer was spun at 500
rpm.
[0087] Removal of any polymer residue from the developed patterns
was accomplished using a PlasmaTherm reactive ion etching (RIE)
system using the following conditions: 5 standard cubic centimeters
per minute (sccm) of CHF.sub.3, 45 sccm of O.sub.2, 250 milliTorr
(mTorr), 300W, 35.degree. C. The etching rate of the polymer under
these conditions is approximately 300 nm/min. Plasma enhanced
chemical vapor deposition (PECVD) was performed to deposit a
SiO.sub.2 overcoat for encapsulation of the polymer channel
patterns. The SiO.sub.2 was deposited with a PlasmaTherm PECVD
using the following conditions: 380 kHz RF frequency, 50W power,
200.degree. C., 550 mTorr, and a gas mixture of N.sub.2O (1400
sccm) and 2% SiH.sub.4 diluted in N.sub.2 (400 sccm). The
deposition rate for the oxide using these conditions is
approximately 50 nm/min.
[0088] Thermal Decomposition Program
[0089] For the thermal decomposition process, the fractional
decomposition can be calculated from the TG curve as shown in
equation (1): 2 = W 0 - W W 0 - W f ( 1 )
[0090] where W.sub.0 is the initial mass, W is the mass remaining
at some time during the decomposition, and W.sub.f is the final
mass of the sample at the end of the thermal cycle. The kinetic
description for thermal decomposition of the polymer is generally
expressed as shown in equation (2): 3 t = k ( 1 - ) n = A exp ( - E
a R T ) ( 1 - ) n ( 2 )
[0091] where n is the overall order of decomposition reaction, A is
the Arrhenius pre-exponential factor, and E.sub.a is the activation
energy of the decomposition reaction.
[0092] In order to avoid a sudden and large release of the gaseous
decomposition products from the polymer patterns that may result in
distortion of the channel structure, it is desired to keep the
decomposition rate 4 ( t )
[0093] constant during the entire decomposition process. Assuming
the decomposition rate is equal to a constant, r, throughout the
decomposition process then: 5 t = r , and , t = 0 , = 0 ( 3 )
[0094] Integrating equation (3) gives the general desired result
shown in equation (4):
a=rt (4)
[0095] Assuming that the reaction order, activation energy, and
pre-exponentional factor do not change significantly during the
decomposition, d.alpha./dt and .alpha. can be replaced with r and
rt respectively in equation (2) which results in the following
equation: 6 r = A exp ( - E a R T ) ( 1 - r t ) n ( 5 )
[0096] It is now possible to rearrange equation (5) to solve for
the necessary temperature versus time profile that is required to
maintain a constant rate of polymer decomposition throughout the
entire process. The explicit expression for temperature versus time
is shown in equation (6). 7 T = E a R [ ln A ( 1 - r t ) n r ] - 1
( 6 )
[0097] Thus, in order to design a heating profile it is necessary
to specify four parameters: the three kinetic parameters (A,
E.sub.a and n) that describe the polymer decomposition, and r the
desired polymer decomposition rate. Based on regression of TGA data
performed in previous experiments, the kinetic parameters for the
polymer used here were determined to be: A=5.8.times.10.sup.14
min.sup.-1, E.sub.a=207 kJ/mol and n=1.05. Thus, for a given
constant decomposition rate, r, one can obtain a curve of
temperature versus decomposition time.
[0098] Results and Discussion
[0099] Decomposition Condition: Thermal decomposition of the
photodefinable sacrificial polymer was performed in a pure nitrogen
atmosphere in order to avoid any oxidation of the polymer that
could result in the formation of non-volatile decomposition
products and undesirable residue in the microchannels. In addition
to using an inert atmosphere, as suggested previously a controlled
heating profile was used to maintain a relatively constant polymer
decomposition rate. This constant decomposition rate ensures that
gaseous products are not released at such a rate that high
pressures are generated that significantly deform the channel
shape.
[0100] FIG. 6 illustrates curves of the decomposition rate versus
time for pure PNB samples decomposed at both a constant temperature
of about 425.degree. C. (isothermal decomposition) and various
heating rates (dynamic decomposition), respectively. In each case,
there is a peak in the decomposition rate. The width of the peak
corresponds to the transition period during the conversion of
sacrificial polymer to gaseous products. Higher heating rates or
higher temperature isothermal decompositions result in a sharp peak
in the decomposition rate profile. This implies that the majority
of the decomposition process occurs over a short time interval,
thus resulting in a sudden and large release of the gaseous
decomposition products. It was therefore expected that controlling
the decomposition rate at a constant low level using controlled
heating profiles could eliminate this phenomena, and thus prevent
channel distortion during decomposition. It was decided to test
this theory by comparing the effect of various decomposition
procedures on the final resulting microchannel shapes and
sizes.
[0101] Based on equation (6), the temperature versus time heating
profiles required to achieve decomposition rates of 1, 2, and 3%
per minute were calculated and are illustrated in FIG. 7. The
figure illustrates that, at a constant decomposition rate, the
decomposition temperature during most of decomposition time should
be set to a relatively low temperature, with a slight ramp rate.
However as the decomposition nears completion, higher temperatures
can be used which helps obtain complete decomposition of the
polymer within a reasonable time.
[0102] Representative temperature profiles that closely approximate
the smooth temperature versus time curves produced via equation (6)
were used to perform the decompositions. FIG. 8 illustrates the
temperature versus time curve calculated using equation (6) and the
corresponding simple mimic heating profile that was tested in the
Lindberg decomposition furnaces for device fabrication. FIG. 9
illustrates TGA results for the simple mimic heating program that
was designed to achieve a 1%/minute decomposition rate. The DTG
curve demonstrates that the decomposition rate does indeed
fluctuate closely around the desired 1%/minute level without
extreme variations. Thus, the sharp peak in the decomposition rate
shown in FIG. 6 can be avoided by using more intelligent heating
profiles (a non-linear heating profile as a function of time). When
this same mimic heating profile is used in processing encapsulated
polymer samples, no distortion in the encapsulated channels was
observed but electron microscopy revealed that small amounts of
polymer residue were left in the channel structures. Two different
modifications to the mimic heating profile were tested in an
attempt to remove this residual polymer. In the first case, a final
hold at 455.degree. C. for one hour was used in an attempt to
remove the residual polymer. This high temperature hold did indeed
reduce the residual remaining polymer substantially as observed in
SEM cross sections, but some remaining residue was left even after
the one hour hold. A second method that involved doubling the
intermediate holds shown in FIG. 8 was also tested. This
effectively reduced the average decomposition rate even further, to
somewhere approaching the 0.5%/minute level. In this case, it was
observed that no distortion of the channel profile occurred during
the decomposition and essentially no polymer residue was found in
the microchannel after decomposition. This suggests that there may
be additional byproducts formed during the decomposition if the
process is ramped too quickly. This results in a residue that can
be difficult to remove, even with high temperature processing.
Longer holds at lower temperatures can be used to both slow the
decomposition rate (and thus reduce pattern profile distortion) and
to eliminate residual polymer in the final channel structures.
[0103] Microchannels encapsulated with polyimide and SiO.sub.2:
Microchannels have been made following the scheme in FIGS. 3A
through 3F. In the processing, about 3.5 to 4.0 .mu.m thick PNB/PI
film (4 wt % initiator in PNB) was cast using a spin speed of 2400
rpm and softbake condition of 110.degree. C./60 seconds. The film
was exposed to UV light using a chrome on quartz mask with dose of
450 mJ/cm.sup.2 and post-exposure baked at 120.degree. C. for 30
minutes in an oven. After post-exposure baking, the film was
spray-developed using xylene to produce the desired channel
patterns. There was no noticeable residue remaining after
development in the patterned areas, but direct overcoating of the
encapsulant material on the as-developed features resulted in poor
adhesion to the substrate. In fact, small bubbles were observed in
the overcoat materials in the areas where the sacrificial polymer
was presumably developed cleanly away from the substrate.
Therefore, it is possible that some small amount of polymer residue
remains after development that prevents good adhesion of the
overcoat to the substrate.
[0104] In order to avoid this phenomenon, a residue removal
treatment was employed by dry-etching in an oxygen plasma using an
RIE before the channel patterns are encapsulated. After residue
removal using the plasma, samples were then encapsulated using
either polyimide or SiO.sub.2. Polyimides are good materials for
encapsulation because they display high glass transition
temperatures and thermal stability, low dielectric constant,
modulus, moisture adsorption and stress. Here, HD Microsystems PI
2734 polyimide, was used to overcoat some of the channel
structures. In these cases, the PI 2734 was spin-coated on the top
of the channel patterns at a speed of 2300 rpm for 30 sec, and
cured at 350.degree. C. for 1 hr under N.sub.2. The thickness of
the polyimide layer under these conditions is approximately 4.5
.mu.m. In addition, some channel structures were encapsulated using
SiO.sub.2. In these cases, a 2-.mu.m thick encapsulation layer of
SiO.sub.2 was deposited using the PECVD recipe described
earlier.
[0105] The decomposition of the encapsulated polymer patterns was
performed at various decomposition rates to investigate the effect
of the rate on the channel structure. FIGS. 10A through 10B
illustrate SEM images of the channel encapsulated with polyimide
and decomposed at different rates using different heating profiles.
The results indicate that the decomposition rate does indeed affect
the channel structure significantly. At low decomposition rates (1
or 2%/minute), the channel structures produced maintain the size
and shape of the original PNB sacrificial polymer pattern. However,
at relatively high decomposition rates (3%/min) or when a high
constant temperature decomposition process is used, the
microchannels are distorted into dome- or arc-shaped profiles. It
is also obvious that this distortion problem becomes a more
important issue for microchannels as their lateral size increases.
Channels with larger widths clearly deformed more than channels of
smaller dimensions. SEM images of channels encapsulated with
SiO.sub.2 are shown in FIGS. 11A through 11F. It was observed that
the extent of channel deformation appears to be higher in the
SiO.sub.2 overcoated structures as compared to the polyimide
overcoated channels at the same nominal channel feature sizes and
polymer decomposition rates. This larger deformation in the
SiO.sub.2 overcoated samples could be due to both differences in
the mechanical properties of the two overcoat materials and
differences in the diffusion rate of the decomposition products
through the overcoat materials.
[0106] Microchannels with tapered cross-section structure: In order
to fabricate the tapered microchannel structures, the concept
described here is to use a lithography process employing a
gray-scale photomask and a low contrast photosensitive sacrificial
material. A series of experiments was performed to investigate the
possibility of using such an approach for producing microchannels
that are shaped in a controlled manner in all three dimensions.
[0107] Channel features were designed with an approximately linear
gradient in percent transmission across the width of the channel
with varying ratios of chrome stripes to clear, transparent area.
In this particular case, the chrome stripe features were designed
to be 200 nm in size and thus served as sub-resolution features for
the photosensitive sacrificial polymers used in this work. Two
masks were fabricated from these designs by electron beam
lithography at ETEC Systems (Hayward, Calif.). Table 2 describes
the two main channel features used in this work in more detail.
Using this type of gray-scale mask allows for the photosensitive
sacrificial material to be exposed to a range of doses across the
width of the channel feature using a single lithographic exposure
step. This exposure gradient in conjunction with a low contrast
resist material can be used to produce a feature that is shaped in
both the lateral and vertical directions with respect to the plane
of the substrate in a single lithographic process.
[0108] Two photosensitive materials with different contrast levels
were used to generate tapered microchannel structures with this
mask. FIG. 12 illustrates the contrast curves for the two
photosensitive sacrificial polymer formulations used in this work.
The methods of measuring contrast curves and calculating contrast
values for these materials have been discussed previously in the
literature. The contrast factors for these two systems are a modest
0.51 and 0.85 for the 2 wt % (referred to as "material 1") and 4 wt
% (referred to as "material 2") photoinitiator relative to dry
polymer loadings, respectively.
2TABLE 2 Characteristics of the gray-scale microchannel photomask
Feature I Channel Width 60 .mu.m Zone Size: 6 .mu.m 6 .mu.m 6 .mu.m
. . . 6 .mu.m 6 .mu.m 6 .mu.m Transparency (TP): 100% 90% 80% . . .
30% 20% 10% Feature II Channel Width 80 .mu.m Zone Size: 4 .mu.m 4
.mu.m 4 .mu.m . . . 4 .mu.m 4 .mu.m 4 .mu.m Transparency (TP): 100%
95% 90% . . . 15% 10% 5%
[0109] Using this contrast curve data, it is possible to calculate
a rough prediction of the pattern profile that will result from
exposure using a gray-scale mask with these photosensitive
materials if the relative transparency as a function of position on
the mask is known accurately. Based on polynomial fitting, the
contrast curves can be adequately described using the following
functions:
f.sub.1=0.0236[log(D)].sup.3-0.357[log(D)].sup.2+1.818[log(D)]-2.13
(7)
f.sub.2=0.0352[log(D)].sup.3-0.653[log(D)].sup.2+2.95[log(D)]-2.92
(8)
[0110] Here f.sub.i is the fraction of the film thickness remaining
after exposure to a dose D and wet development for material i.
[0111] An approximate shape of the channel patterns that will be
produced from a gray-scale mask can thus be predicted using Eq.
9,
d(x)=f.sub.1(log[D.multidot.TP(x)]).multidot.FT, (9)
[0112] where, f.sub.i is the contrast function for material i, d(x)
is the thickness of the film (after development) at a certain
position x across the channel pattern, TP(x) is the fractional
transparency of the mask at the position x across the feature, D is
the nominal exposure dose used, and FT is the original thickness of
the cast film. The outline of the simulated channel pattern include
the points calculated by Eq. 9, which were then smoothed by
seven-point smoothing, Eq. 10. 8 S i = Y i - 3 + 2 Y i - 2 + 3 Y i
- 1 + 4 Y i + 3 Y i + 1 + 2 Y i + 2 + Y i + 3 16 ( 10 )
[0113] where S.sub.i and Y.sub.i are the smoothed signal and
original signal for the i.sup.th point respectively.
[0114] Tapered-structure channel patterns were fabricated using the
gray-scale lithographic approach using a sequence of steps similar
to those outlined in FIGS. 3A through 3F. First, 12-.mu.m thick
PNB/PI films were cast using a spin speed of 700 rpm and softbake
condition of 110.degree. C. for 2 minutes. The films were then
exposed to UV light with the gray-scale mask. The nominal exposure
dose was set using the contrast curve data for the photosensitive
material to obtain a film with 80% original thickness remaining
after development under a 100% transparent feature. The doses used
were 1300 mJ/cm.sup.2 and 165 mJ/cm.sup.2 for 2 wt % and 4 wt %
initiator loadings, respectively. The films were post-exposure
baked at 120.degree. C. for 30 minutes in oven. The films were
spray developed using xylene at a spin speed of 500 rpm for 30
seconds. The final shape of the microchannel patterns was measured
using profilometry.
[0115] FIGS. 13 and 14 illustrate the real Feature I type PNB
patterns produced as measured by profilometry, and for comparison
the predicted microchannel patterns (using equations 7 through 10),
for the systems with 2 wt % and 4 wt % initiator loadings. A
comparison between different patterns produced by the two
formulations clearly shows that the material with the lower
contrast produces a profile that more closely resembles the desired
smoothly tapered structure. However, it can be seen that the simple
prediction of the profile shape only roughly approximates the
actual feature produced using this method. Upon closer inspection
of the mask, it was apparent that the desired smooth gradient in
transmission was not faithfully reproduced into the mask due to the
extremely small feature sizes used for the constituent patterns.
This brings up the issue that accurate gray scale mask production
for such a method may in fact be a challenging task. In any case,
with more careful attention and accurate transfer of the design to
the mask, it should be possible to use the contrast data for a
material in conjunction with equations (7) through (10) to design a
gray-scale mask feature for a specific photosensitive material that
can be used to obtain any desired pattern shape.
[0116] The tapered polymer microchannel patterns were next
overcoated and decomposed in order to test the ability to transfer
the tapered profile into the final microchannel. First, any polymer
residue was removed from the substrate using an oxygen RIE plasma
etch. The channel patterns were then encapsulated with SiO.sub.2
using the same conditions described previously. The thermal
decomposition of encapsulated channel patterns was performed under
N.sub.2 with a decomposition rate of 0.5%/minute. SEM images of the
resulting tapered microchannels are shown in FIGS. 15A through 15D.
Due to the ability to carefully control the decomposition rate of
the polymer by controlling the heating profile during
decomposition, no deformation was observed in the channel
structure. This can be seen by comparing the profiles of the
original PNB patterns in FIGS. 13 and 14 with the SEM channel cross
sections in FIGS. 15A through 15D. The widths of the channels in
FIGS. 15A through 15D are narrower than the feature sizes on the
gray-scale mask due in part to slight RIE over-etching during the
polymer residue removal step. Comparing FIGS. 15A through 15B with
FIGS. 15C through 15D, it can be seen that a low contrast
sacrificial material is desirable for the fabrication of smoothly
tapered microchannel structures. The right hand side of FIGS. 15A
through 15C, and left hand side of FIGS. 15B through 15D are
non-gray scale features, for reference. As expected, the final
shape of the channel structure is determined by a combination of
the gray-scale pattern on the mask, the contrast of the
photosensitive material, and the nominal exposure dose used in
printing the feature.
[0117] In order to obtain an idea of the effectiveness of
fabricated channel cross sections in reducing dispersion in flow
around microchannel corners, the expected fluid transit times
around a corner of the shape shown in FIG. 13 were simulated using
FLUENT as described previously. FIG. 16 illustrates the predicted
transit times for flow around this corner using the boundary
conditions and velocities used in the earlier idealized channel
simulations. It is clear from this simulation that even the crudely
shaped channel fabricated for demonstration purposes in this work
would be expected to perform better than the standard rectangular
cross section channel. Further, it is hoped that by optimizing mask
design and process conditions, that a more ideal shape similar to
that shown in FIG. 5C can be achieved and used for device
fabrication.
[0118] Conclusions
[0119] The fabrication of microchannels has been demonstrated by
using photosensitive sacrificial polymer materials. The process
consists of patterning the sacrificial polymer via
photolithography, removal of polymer residue using RIE,
encapsulation with a dielectric medium, and thermal decomposition
of encapsulated polymer channel patterns. A method for designing
heating programs to keep the thermal decomposition of sacrificial
polymer at a constant rate was presented using the kinetic model of
polymer decomposition. Heating programs designed using this
approach have been demonstrated to prevent sudden and high
decomposition rates (e.g., those which result in drastic release of
gaseous decomposition products that distort channel features), and
were also shown to produce microchannel patterns with well
controlled shapes that do not exhibit any substantial deformation
after the thermal decomposition of the sacrificial polymer.
Controlling the decomposition rate and slowly releasing the gaseous
decomposition products allows the decomposition products to
permeate through the overcoat at a rate roughly equivalent to the
decomposition rate, and thus avoids the build-up of high pressures
in the microchannel which can lead to distortion and failure of the
structure. It was also found that larger channels have a greater
tendency toward distortion. A gray-scale lithographic process has
been developed and demonstrated for the production of microchannels
with tapered cross-sections. Such tapered channels have been shown
through simulation to be able to reduce effects such as dispersion
that are detrimental to microfluidic system performance.
[0120] It should be emphasized that the above-described embodiments
of this disclosure are merely possible examples of implementations,
and are set forth for a clear understanding of the principles of
this disclosure. Many variations and modifications may be made to
the above-described embodiments of this disclosure without
departing substantially from the spirit and principles of this
disclosure. All such modifications and variations are intended to
be included herein within the scope of this disclosure and
protected by the following claims.
* * * * *